A doctor has released new research which questions whether Attention Deficit Hyperactivity Disorder (ADHD) causes sleep problems - or is itself caused by sleeping problems.

ADHD is a group of behavioural symptoms that include hyperactivity, mood swings and impulsiveness.

Symptoms tend to be noticed at an early age and may become more noticeable when a child's circumstances change, such as when they start school. Most cases are diagnosed when children are 6 to 12-years-old.

There out of four ADHD patients suffer from sleep problems, but previously both health issues were thought to be separate problems.

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Now Doctor Sandra Kooij, Associate Professor of Psychiatry at VU University Medical Centre in Amsterdam, has suggested a link between ADHD and sleep disorders.

She said that in three out of four ADHD patients, changes in the level of the sleep hormone Melatonin are delayed by 1.5 hours and changes in body temperature, associated with sleep, are delayed.

She has also shown that an array of sleep-related disorders are linked to ADHD, including restless-leg syndrome, sleep apnea - where breathing is interrupted - and delayed sleep phase syndrome, a disorder where people go to bed and get up earlier than average.

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She pointed out that many ADHD patients show greater alertness in the evening and benefit from taking Melatonin around this time, in order to help them sleep. They also benefit from exposure to bright light in the morning to help them wake up.

Dr Kooij said: "There is extensive research showing that people with ADHD also tend to exhibit sleep problems.

"What we are doing here is taking this association to the next logical step: pulling all the work together leads us to say that, based on existing evidence, it looks very much like ADHD and circadian problems are intertwined in the majority of patients.

"We believe this because the day and night rhythm is disturbed, the timing of several physical processes is disturbed, not only of sleep, but also of temperature, movement patterns, timing of meals, and so on.

"If you review the evidence, it looks more and more like ADHD and sleeplessness are two sides of the same physiological and mental coin."

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She also laid out evidence showing that the eyes of 70 per cent of people with ADHD are oversensitive to light, leading many to wear sunglasses during the day - which could reinforce problems with the sleep cycle.

She said that chronic late sleep leads to sleep debt which is linked to obesity, heart disease and cancer. She claimed that these health problems could be in part prevented by resetting ADHD patients' sleep patterns.

"If the connection is confirmed, it raises the intriguing question: does ADHD cause sleeplessness, or does sleeplessness cause ADHD? If the latter, then we may be able to treat some ADHD by non-pharmacological methods, such as changing light or sleep patterns, and prevent the negative impact of chronic sleep loss on health."